CDC Reports 279 Pregnant Women With Zika in U.S.

Nancy Trinidad, who is 32 weeks pregnant, listens to the explanation of a doctor about how to prevent Zika, Dengue and Chikungunya viruses at a public hospital in San Juan, Puerto Rico, February 3, 2016. REUTERS/Alvin Baez ALVIN BAEZ / Reuters

More than 270 pregnant women in the U.S. are infected with the Zika virus and worry about whether their babies will be born with birth defects, federal health officials announced Friday.

The Centers for Disease Control and Prevention updated the way it reports Zika-affected pregnancies, and said the new numbers show 279 women who tested positive for the virus.

So far, fewer than a dozen have had an "adverse event," such as a miscarriage or evidence that the fetus has a birth defect, CDC officials said.

"These new numbers reflect a broader group of pregnant women — pregnant women who have any laboratory evidence of possible Zika virus infection, and whether or not they recalled symptoms — compared with numbers previously reported," the CDC said in a statement.

The numbers include women who have already given birth, those who have miscarried and those who may have had abortions because of birth defects.

The CDC has been recommending for months that any woman who's pregnant who thinks she could have been exposed to the virus get tested. This includes women who have traveled to Zika-affected areas, and women whose sexual partners have.

The virus is transmitted mostly by mosquitoes, but it can be sexually transmitted, also.

What has not been clear is whether the fetus of a woman who is infected but never had any symptoms is likely to have a birth defect. So CDC had only reported some of the cases of pregnant women with Zika.

But some studies have suggested that a fetus can be affected even if the mother never knew she had Zika. The virus often doesn't cause symptoms. The CDC is now counting all women who have tested positive, and reporting those numbers publicly.